var sun = new Image();
var moon = new Image();
var earth = new Image();

function draw() {
    sun.src = 'https://mdn.mozillademos.org/files/1456/Canvas_sun.png';
    moon.src = 'https://mdn.mozillademos.org/files/1443/Canvas_moon.png';
    earth.src = 'https://mdn.mozillademos.org/files/1429/Canvas_earth.png';
    window.requestAnimationFrame(drawAnimate);
}

function drawAnimate() {
    var ctx = document.getElementById('tutorial').getContext('2d');
    ctx.globalCompositeOperation = 'destination-over';
    ctx.clearRect(0, 0, 300, 300);
    ctx.fillStyle = 'rgba(0, 0, 0, 0.4)';
    ctx.strokeStyle = 'rgba(0, 153, 255, 0.4)';
    ctx.save();
    ctx.translate(150, 150);

    // Earth
    var time = new Date();
    ctx.rotate(((2 * Math.PI) / 60) * time.getSeconds() + ((2 * Math.PI) / 60000) * time.getMilliseconds());
    ctx.translate(105, 0);
    ctx.fillRect(0, -12, 50, 24); // Shadow
    ctx.drawImage(earth, -12, -12);

    // Moon
    ctx.save();
    ctx.rotate(((2 * Math.PI) / 6) * time.getSeconds() + ((2 * Math.PI) / 6000) * time.getMilliseconds());
    ctx.translate(0, 28.5);
    ctx.drawImage(moon, -3.5, -3.5);
    ctx.restore();

    ctx.restore();
    ctx.beginPath();
    ctx.arc(150, 150, 105, 0, Math.PI * 2, false); // Earth orbit
    ctx.stroke();

    ctx.drawImage(sun, 0, 0, 300, 300);

    window.requestAnimationFrame(drawAnimate);
}
